Among all described serotonin (5-HT) receptors in mammals, the type three (5-HT3) is the only ligand-gated ion channel receptor for serotonin. By using double in situ hybridization histochemistry, we found co-expression of the functional 5-HT3A subunit of the 5-HT3 receptor and the central CB1 cannabinoid receptor in neurons of the rat telencephalon. Double-labeled 5-HT3A/CB1 neurons were found in anterior olfactory nucleus, superficial and deep layers of cortex, hippocampal formation (hippocampus, dentate gyrus, subiculum and entorhinal cortex) and amygdala. Analysis of the proportion of neurons co-expressing 5-HT3A and CB1 receptors in cortex and amygdala showed that depending on the brain region, 37-53% of all neurons expressing the 5-HT3A subunit also express CB1 transcripts; while 16-72% of the total population of neurons expressing CB1 mRNA co-express the 5-HT3A subunit. By using a combination of double in situ hybridization and immunohistochemistry, we demonstrated that 5-HT3A/CB1-expressing neurons contained the inhibitory neurotransmitter g-amino butyric acid (GABA). These results imply that in distinct regions of the telencephalon, GABA neurons that react to cannabinoids may also be responsive to serotonin through 5-HT3 receptors. Cellular co-existence of 5-HT3A and CB1 transcripts in interneurons of cortex, hippocampal formation and amygdala suggest possible interactions between the cannabinoid and serotonergic systems at the level of GABA neurotransmission in brain areas involved in cognition, memory and emotion.